1. Field of Invention
The present invention relates to a projection display device provided with a light synthesizing means such as a cross-dichroic prism.
2. Description of Related Art
A cross-dichroic prism is sometimes employed in a projection display device that projects a color image onto a projection screen. A cross-dichroic prism is an optical element that synthesizes light of three colors, red, green, and blue, and emits the light in the same direction. Such a projection display device is disclosed in, for example, Japanese Unexamined Patent Publication No. 1-302385.
FIG. 16 is a conceptual view showing the principal parts of a projection display device. This projection display device includes three liquid crystal light valves 42, 44, and 46, a cross-dichroic prism 48, and a projection lens system 50. The liquid crystal light valves 42, 44, and 46 are each composed of a liquid crystal panel sandwiched between two polarizers placed so that the polarization axes thereof intersect at right angles. The cross-dichroic prism 48 synthesizes light of three colors, red, green, and blue, modulated by the three liquid crystal light valves 42, 44, and 46, and emits the synthesized light toward the projection lens system 50. The projection lens system 50 focuses the synthesized light onto a projection screen 52.
As shown in FIG. 16, a red reflecting film 61 and a blue reflecting film 62 are located in the form of an X in the cross-dichroic prism 48. The red reflecting film 61 and the blue reflecting film 62 each may be formed as a dielectric multilayer film that has a high reflectance for s-polarized light with a wider range of wavelengths than for p-polarized light. In the above projection display device disclosed in Japanese Unexamined Patent Publication No. 1-302385, such a red reflecting film 61 and a blue reflecting film 62 are used, and the red and blue light of the light of three colors, which is incident on the cross-dichroic prism 48, is made incident as s-polarized light and the green light is made incident as p-polarized light, which improves the reflection characteristic of the cross-dichroic prism 48. Specifically, when red light and blue light polarized in random directions are caused to enter the liquid crystal light valves for red and blue light 42 and 46, respectively, s-polarized red and blue light are emitted therefrom and caused to enter the cross-dichroic prism 48. Here, the general property of the liquid crystal light valve is employed, it being that only predetermined linearly polarized light is emitted. On the other hand, when green light polarized in random directions is caused to enter the liquid crystal light valve for green light 44, p-polarized green light is emitted therefrom and caused to enter the cross-dichroic prism 48.
Since light polarized in random directions is incident on three liquid crystal light valves in the above-mentioned conventional projection display device, components polarized in the direction orthogonal to the direction of the polarized components to be transmitted are absorbed by the liquid crystal light valves 42, 44, and 46. As a result of such light absorption, heat is generated in the liquid crystal light valves 42, 44, and 46, affecting the liquid crystal light valves and adjacent optical elements. This also applies to light valves other than such liquid crystal light valves.
Moreover, this also applies to a case in which the cross-dichroic prism 48 is replaced by a cross-dichroic mirror composed of two dichroic mirrors, both of which include the red reflecting film 61 and the blue reflecting film 62, located so that they cross in about the form of an X.